The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
One of the performance criteria for oscillators is frequency stability. The application the oscillator is used in will determine the extent to which short term and/or medium to long term frequency stability is required.
A dielectrically loaded cavity resonator is sensitive to temperature fluctuations in the cavity resonator. As the temperature of the cavity resonator changes, the dielectric constant of the dielectric material in the cavity resonator will also change according to its thermal coefficient of permittivity. This results in corresponding changes in the resonant frequency of the cavity resonator. Where an oscillator uses the dielectrically loaded cavity resonator as its frequency determining element, temperature fluctuations in the cavity resonator will affect the frequency stability of the oscillator because the operating frequency of an oscillator using the dielectrically loaded cavity resonator will also change with temperature.
In an attempt to reduce the effects of this phenomena, many prior art oscillators use thermal control systems to hold the cavity resonator within a narrow temperature range to improve the oscillator's frequency stability. However, such systems are still prone to long term frequency drift resulting from temperature changes within the controlled range.
Many dielectric materials are anisotropic and accordingly have multiple thermal coefficients of permittivity, corresponding to the axes of the dielectric material's crystal structure. For example, sapphire has two thermal coefficients of permittivity, TCPz and TCP⊥.
There have been attempts at reducing the effects of an anisotropic dielectric material's thermal coefficients of permittivity on the frequency stability of an oscillator using the dielectric material. Such systems typically operate the cavity resonator in two modes simultaneously—a transverse magnetic (“TM”)-mode and a transverse electric (“TE”)-mode, making use of the anisotropy of the thermal coefficient of permittivity in the dielectric material to give an indication of frequency drift. The operating frequencies of the TM-mode and the TE-mode are chosen to be different by a small amount, so that when the two signals are mixed together, the result is a lower frequency signal that is compared to a reference signal at the same lower frequency to provide an indication of the frequency drift of the oscillator. For example, for an oscillator operating at 9 GHz, two modes separated by 79 MHz may be used, such as 9.000 GHz and 9.079 GHz. The lower frequency 79 MHz signal, produced by mixing the 9.000 GHz and 9.079 GHz signals together, is then mixed with a local 79 MHz source to produce an error signal, which is then used as a control signal in a feedback system to reduce the frequency drift of the oscillator and consequently improve the oscillator's frequency stability. One drawback of this system is that frequency stability of the local source limits the frequency stability of the oscillator.